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Subject:	Re: QFN with solder slug
From:	"David D. Hillman" <[log in to unmask]>
Reply To:	TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:	Thu, 1 Sep 2011 07:05:08 -0500
Content-Type:	text/plain
Parts/Attachments:	text/plain (214 lines)
Hi Frank - as you detailed, there are a couple of ways to skin this cat. 
You can use soldermask tenting if you setup your process and have good 
soldermask design rules. I have not had much luck with soldermask tenting 
due to fluids entrapment problems so we fill and plate shut the vias. I am 
able to avoid the fluids entrapment concerns and maintain the thermal 
and/or electrical grounding expectations of some of the circuit designers. 
A big portion of the answer is your relationship with your board 
fabricator and the type of soldermask used in your designs. Good details 
on how soldermask tenting can be utilized.

Dave



Frank Kimmey <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
08/31/2011 04:32 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Frank Kimmey <[log in to unmask]>


To
<[log in to unmask]>
cc

Subject
Re: [TN] QFN with solder slug






Wow, I hate to disagree with guys like Dave and Dewey, but "it depends" 
(Diet Dew is on the way, Doug) on many factors.
Final finish chemistries can make tenting a bad thing, plugging can be 
expensive and too large diameter will cause solder starvation.
We do lots of these and make sure all holes are small enough to minimize 
wicking with RoHS solders.
We have also found a little solder starvation (as long as there is still 
good wetting) doesn't cause to many reliability concerns.
We use small (smaller does not transfer heat as well but reduces wicking) 
vias  under the part and as many large vias as can be fit around the part.
Thermal relief sorta cancels part of the value so everything gets flooded.
Also, tie to every layer possible, inner layers do not transfer heat real 
well but every little bit helps.
It is doable without extra fab/assy costs but may require some 
experimentation with your suppliers to find the right combination.
Good luck,
FNK

Frank N Kimmey CID+
Manager - PCB Design
Powerwave Technologies Inc.
Mobile - 916-670-0645


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey 
(EHCOE)
Sent: Wednesday, August 31, 2011 2:20 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug

Vias are like puns (Dewey"isms"); they should all be in"tent"ed (capped, 
filled, plated-over). Thanks for agreeing.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, August 31, 2011 2:12 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug

Hi Mark - if you leave the thermal vias open then the solder will thieve

down them leaving the center pad "solder starved". I recommend you use 
some fabrication format which permits the vias not allowing solder to 
thieve - aka either capping them or filling them. The IPC-7093 standard on 
Bottom Termination Components also may have some helpful information.

Dave Hillman
Rockwell Collins
[log in to unmask]




Mark Larson <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
08/31/2011 02:52 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Mark Larson 
<[log in to unmask]>


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<[log in to unmask]>
cc

Subject
[TN] QFN with solder slug






I am curious how others are handling these parts for soldering. Do you 
leave the thermal vias in the slug covered or uncovered with solder
mask? 
Do you fill with anything? What size hole on a 62 mil board? Tie to all 
GND planes or just the back side? Thermal relief the via or bury it in
the 
plane?

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